1. Field of the Invention
This invention relates to a golf ball comprising a solid core and a two-layer cover, and more particularly, such a golf ball which is improved in restitution, feel and control at no sacrifice of durability.
2. Prior Art
Mixtures of balata rubber and elastomers or plastic materials are known as the cover stock of golf balls. Such balata rubber covers are relatively soft and flexible. Golf balls using soft balata rubber covers are more forced against the club face upon impact to produce a high spin. The balata covered balls are advantageous in that an experienced golfer can apply a spin to the ball to control the ball in flight to produce a draw or a fade, or a backspin to cause the ball to "bite" or stop abruptly on contact with the green upon an approach shot. Additionally, the balata covered balls give soft feeling to beginner golfers. The spin and feeling inherent to the balata cover balls are important to slow swing play with a short iron and favored especially by experienced golfers.
Despite the above-mentioned advantages of balata rubber, the balata-covered balls are easily cut if mishit. Therefore, golf balls produced with balata rubber or balata rubber-containing cover compositions have a relatively short life span.
Due to this negative property, balata rubber and its synthetic substitutes, trans-polybutadiene and trans-polyisoprene have today been replaced essentially by new cover stocks, primarily by a family of ionomer resins. The ionomer resins are polymers having an ionic bond between linkage chains. Because of their durability and resilience, the ionomer resins have become more important as the golf cover stock than the prior art balata rubber.
In general, ionomer resins are ionic copolymers comprising an olefin such as ethylene and a metal salt of an unsaturated carboxylic acid such as acrylic acid, methacrylic acid and maleic acid. Some of the acid groups in the copolymer are neutralized with metal ions, for example, sodium and zinc. The ionomer resins have been used as a thermoplastic resin cover material exhibiting superior durability, resilience and other properties to the balata rubber. However, when the ionomer resins are used as the golf ball cover stock, the balls become more durable, but are very hard and lack enough flexibility to impart a spin rate necessary to control the ball in flight. That is, since the ionomer resins are harder than the balata rubber, the ball is not so strongly forced against the club face upon impact and receives less spin for this reason. In addition, the harder ionomer resins lack the feeling inherent to the soft balata rubber.
It was thus desired to have a golf ball cover stock which possesses not only the impact resistance and resilience given by the ionomer resin, but also the spin and feeling inherent to the soft balata rubber winning experienced golfers' favor.
To solve the problem, many two- and three-piece golf balls have been proposed. One attempt to improve resilience, durability and spin is a multi-layer cover comprising one or more ionomer resins. The multi-layer golf ball is prepared by molding an inner cover layer on a core and adding an outer cover layer thereon. The inner cover layer is constructed of a hard, high modulus resin material such as Surlyn 8940 which is a sodium ionic low acid content ionomer resin (methacrylic acid content: less than 15% by weight). Molded around the inner cover layer is the outer cover layer which is constructed of a relatively soft, low modulus resin material such as Surlyn 9020 which is a zinc ionic low acid content ionomer resin. The multilayer structure is obtained in this way.
The hard, high modulus resin used in the inner cover layer provides a higher coefficient of restitution than the core while the outer layer of soft, low flexural modulus resin gives spin and feeling comparable to those of balata-covered golf balls. Although these multi-layer golf balls exhibit increased spin performance and feeling, they are not practically acceptable because their scaping resistance and restitution are inferior as compared with two-piece golf balls with a single layer cover.
To enhance the restitution of multi-layer golf balls, the multi-layer cover disclosed in JP-A 343718/1994 uses a high acid content ionomer resin in the inner cover layer. In this proposal, a high acid content ionomer resin containing at least 16% by weight of .alpha., .beta.-unsaturated carboxylic acid is used as the inner cover layer. A relatively soft, low modulus resin material consisting of an ionomer resin, a non-ionomer thermoplastic elastomer or a mixture thereof is molded over the inner cover layer to form an outer cover layer, providing a multi-layer structure.
According to this proposal, the multi-layer golf ball. is improved in restitution by the use of a high acid content ionomer resin in the inner cover layer. Although the ball travels an added distance, it is less durable because of the low impact resistance of the high acid content ionomer resin in the inner cover layer and the difference in hardness between the inner and outer cover layers. Additionally, the use of a hard, high acid content ionomer resin in the inner cover layer causes the ball to give an unpleasant feel upon shots. Due to these drawbacks, the multi-layer golf ball of JP-A 34378/1994 is practically unacceptable.